The present invention relates to a rotor core, a direct-current motor, and a method for winding coils on a rotor core.
Recently, to meet demands for compact and high performance motors, motor cores having an increased coil density and an improved accommodation efficiency are wanted. A rotor core of a direct-current motor has radially extending teeth. A coil is wound about each tooth. A slot between each adjacent pair of teeth, or an accommodation space for a coil, narrows toward the radially inner end and widens toward the radially outer end.
Thus, when winding a coil about each tooth in a concentrated manner, the number of times the coil is wound increases toward the radially outer end. Thus, the measurement of the wound coil along the axial direction of the motor, or the coil height, increases toward the outer end of the teeth, which increases the axial size of the motor.
Japanese Laid-Open Patent Publication No. 9-19095 discloses a rotor core having teeth, in which core the circumferential width of each tooth increases toward the radially outer end, and the height (the measurement along the axial direction of the motor) of each tooth decreases toward the radially outer end. This configuration reduces the coil height at the radially outer sections of the teeth and reduces the size of the motor.
However, in the configuration disclosed in the above publication, the space between the distal sections of each adjacent pair of the teeth widens toward the radially outer end, and each coil is wound about the corresponding tooth using those spaces. Therefore, the space factor of the coils is limited. The distal section of each tooth faces one of magnets provided on a stator. Magnetism converges on the distal section of each tooth. Since the thickness (height) of the distal section, or the magnetism converging section, of each tooth is small, the necessary magnetic flux does not flow through the teeth. This lowers the motor torque.